In public address systems, or in other multi-source systems, it is common to network speakers together. A typical system supplies a relatively high voltage (many use 70.7 V) which is usually stepped or divided down by a sound pressure level (SPL) device.The SPL device limits the power delivered to the speaker, such that the system can be customized to the environment in which it is incorporated. For example, in inherently noisy areas the speaker's power level can be raised to increase the volume of its output. Similarly, in relatively quite areas the speaker's power level can be lowered to decrease the volume of its output.
An impedance matched transformer is one commonly used SPL device. The transformer consists of a primary winding and a secondary winding with several taps. The primary is connected across the supply lines of the system and the speaker is connected to the secondary. The secondary connections are determined by the power requirements of the speaker. This method is relatively expensive and if the speaker is a piezoelectric speaker, an L-C network is formed which may increase the complexity of the compensation circuit. (A piezoelectric speaker's characteristic impedance is substantially capacitive.)
Another commonly used SPL device is a resistor connected in series with the speaker. This method reduces the cost of the system, however, it increases the overall power requirements and if the speaker is a piezoelectric speaker, an R-C network is formed which may again increase the complexity of the compensation circuit.
Therefore, a need exists for a system that incorporates an SPL device that is relatively inexpensive, reduces the overall power requirements, and does not normally increase the complexity of the compensation circuit.